Shift Subnet Backbone Structure in Prysm

[nisdas]

💎 Issue

Background

Since ethereum/consensus-specs#3312 has been merged in for a while, this issue will track the implementation
design and steps of this spec change in Prysm.

Description

Currently each Prysm node will subscribe to a number of attestation subnets proportional to the number of validators it hosts. This puts a strain on nodes running many validators as you would need to process all the attestations flowing through the network. With this change every node in the network will now subscribe to 2 subnets, allowing the load to be distributed across the network. In order to correctly implement this in Prysm we will need to handle a few things:

• Implement Subnet Computation ( based on node-id)
• Disable long-lived subscriptions based on validator count.
• Gate this with a feature flag
• Track peer/subnet discoverability metrics with this feature. With nodes being less 'heavy' , it will
  take more time/effort to find peers with our desired subnets.
• Add support for enforcement of subscribed subnets ( disabled till the next HF)